An ability to detect and measure wavefront tilt and other wavefront aberrations is advantageous in maintaining direction and quality of optical beams. In space-based laser systems, where beam direction and quality are subject to degradation from thermal and mechanical stresses and from spacecraft maneuvering, the ability to measure wavefront tilt and other aberrations is especially useful in generating signals for actuating correction devices.
Interferometric techniques have been developed in the prior art to measure wavefront tilt and other wavefront aberrations for optical beams. For example, U.S. Pat. No. 3,829,219 describes a technique invented by James C. Wyant whereby information about the quality of an optical wavefront passing through an aperture can be otained from an interferogram formed when first order diffraction components produced by two different diffraction gratings are sheared in two mutually orthogonal directions simultaneously. However, the wavefront shearing technique described in the Wyant patent cannot be conveniently used in analyzing an optical beam with a Hartmann-type instrument.
In a conventional Hartmann analysis of an optical beam defined by an aperture, a beam sampling mechanism such as a mask having two or more subapertures covers the beam aperture to form a plurality of subaperture wavefronts. The tilts of the individual subaperture wavefronts are compared with a reference tilt and/or with each other to determine wavefront tilt and other aberrations of the beam. A sampling system for use in performing subaperture wavefront tilt analysis is described in U.S. Pat. No. 4,239,392 to Richard H. Pohle, applicant herein.